Manufacture of large complex devices such as aircraft, ships, and microelectronics require an effect definition of technical requirements. Technical order data (TOD) is a type of requirements definition that can be used for describing aircraft parts, related maintenance procedures, and resource requirements. TOD may use a standard generalized markup language such as ISO 8879:1986 SGML for aircraft part documents. Extensible markup language (XML) is a subset of SGML designed to ease implementation compared to a full SGML parser, and currently more widely used than full SGML.